1. Field of the Invention
This invention generally relates to internal combustion engines, and more particularly to a compression release and vacuum release mechanism for four-stoke cycle engines.
2. Description of the Related Art
Compression release mechanisms for four-stroke cycle engines are well known in the art. Generally, means are provided to hold one of the valves in the combustion chamber of the cylinder head slightly open during the compression stroke while cranking the engine. This action partially relieves the force of compression in the cylinder during starting, so that starting torque requirements of the engine are greatly reduced. When the engine starts and reaches running speeds, the compression release mechanism is rendered inoperable so that the engine may achieve full performance. It is normally advantageous for the compression release mechanism to be associated with the exhaust valve so that the normal flow of the fuel/air mixture into the chamber through the intake valve, and the elimination of spent gases through the exhaust valve is not interrupted, and the normal direction of flow through the chamber is not reversed. Examples of compression release mechanisms for four-stroke engines are numerous and share a common principle which includes activating a valve displacement feature at low crankshaft speeds, i.e., at startup, and deactivating the same at significantly higher crankshaft speeds i.e., run mode.
Presently, conventional four-stoke engines require a significant amount of torque to turn the engine over during the power stroke when combustion is not taking place. This is so because the piston is then moving downwardly against a pressure difference due to increasing suction resulting from the partial discharge of gas from the cylinder during the immediately preceding compression stroke. The increase of torque required corresponds to a substantial operator or starter force required to drive the piston downwardly against that pressure difference.
In response to the torque developed by suction, one prior art combustion engine suggests using a contoured cam lobe which acts to hold the valve open longer between the compression and power strokes. Starting torque was decreased by this mechanism, however compression and accordingly engine power would significantly decrease compared to conventional engines which employ the traditional xe2x80x9cpear-shapedxe2x80x9d cam lobes. Yet another prior art mechanism employed a light spring placed on the stem side of the exhaust valve to hold the valve open during start-up. However, significant intake and exhaust manifold pressures would be required to close the exhaust valve and thus longer times and increased user effort is required to start the engine.
Other devices which compensate for torque caused as a result of suction force developing during the power stroke are disclosed in provisional Patent Application No. 60/231,818, filed Sep. 11, 2000, and Patent Application No. 09/760,953, filed Jan. 15, 2001, both of which are assigned to the assignee of the present application, the disclosures of which are expressly incorporated herein by reference.
The device disclosed in provisional Patent Application No. 60/231,818, utilizes a saddle member pinned to an accessible area of the camshaft and includes a pair of auxiliary cams to sequentially relieve compression and vacuum by lifting the exhaust valve during appropriate portions of the compression and power stroke at engine cranking speeds. The device disclosed in patent application Ser. No. 09/760,953, utilizes an operating member, rotatably fixed along the length of the camshaft, having a compression relieving operating end in engagement with a vacuum release member to sequentially relieve compression and vacuum by lifting the exhaust valve during appropriate portions of the compression and power stroke at engine cranking speeds.
Although effective, the saddle type device is not readily adaptable to some existing engine designs. Traditionally used engine crankcase designs require casting and machining modifications before this release may be implemented. Similarly, the operating shaft type device requires significant additional machining or casting modification to the camshaft to accommodate this release.
Accordingly, it is desired to provide a release mechanism that addresses the significant torque developed by both the compression and power strokes and one that is effective in operation and relatively simple in construction. It is further desired to provide a release mechanism which addresses this significant torque, and is retrofittable to a substantial number of existing engine crankcases without significant modification to the engine.
The present invention overcomes the disadvantages of prior internal combustion engines by providing a mechanical compression and vacuum release, of simple construct, including an operating member reciprocally supported within a camshaft and engaged with a centrifugally activated flyweight wherein movement of the centrifugal flyweight causes radial translation of a vacuum release member through the operating member and the vacuum release member is in lifting engagement with one of the intake or exhaust valves.
A four-stroke internal combustion engine is provided and includes a cylinder block having a cylinder therein and a piston reciprocally disposed within the cylinder. The piston is operably engaged with a crankshaft. At least one intake valve and exhaust valve are reciprocally driven by a camshaft. A vacuum release mechanism includes an operating member reciprocally supported within the camshaft for translation along an axis. A centrifugally actuated flyweight member is engaged with the operating member and rotation of the camshaft above engine cranking speeds causes the flyweight member to move the operating member from a first position to a second position. A vacuum release member is movably supported within the camshaft and in engagement with the operating member wherein translational movement of the operating member causes movement of the vacuum release member. The operating member and flyweight are urged to the first position at engine cranking speeds and are moved by the flyweight member through centrifugal force to the second position at engine running speeds. The vacuum release member is in lifting engagement with one of the valves at the first position during at least a portion of the power stroke of the piston and is out of lifting engagement with one of the valves at the second position.
The present invention further provides a compression release mechanism. The compression release member is movably supported within the camshaft and is in lifting engagement with one of the valves at the first position coinciding with at least a portion of the compression stroke of the piston. The compression release member and the vacuum release member successively attain lifting engagement with one of the valves at the first position and the compression and vacuum release members are out of lifting engagement with one of the valves at the second position.
An object of the present invention is to provide an engine having a mechanical vacuum release mechanism that overcomes substantial operator or starter force caused by suction forces acting on the piston during the power stroke at engine cranking speeds.
Another object of the present invention is to provide a compression and vacuum release mechanism easily retrofittable with existing engines crankcases wherein the release mechanism is disposed within the profile of the existing camshaft assembly. These and other objects, advantages and features are accomplished according to the devices, assemblies and methods of the present invention.